Projection technology makes increasing use of solid state light sources instead of the conventional lamps, e.g. using lasers in a single-chip DLP projector, in three-chip DLP projectors or in other projectors with 3 imagers (LCD, LCoS, . . . ).                Laser-based solid state projectors could be classified in two main categories:        Full laser projectors (using direct red, green and blue lasers)        Laser phosphor projectors (using blue laser to excite a wavelength convertor material to generate some of the three primaries)        
Currently, the full laser projectors are typically ultra-bright projectors aimed at the niche market of digital cinema (DC). Laser phosphor projectors mainly have a lower light output, i.e. under 12,000 lumens and therefore are sold in the markets outside digital cinema. However, recent improvements in the phosphor technology allow laser phosphor projectors to achieve even brightness levels up to 20,000 lumens and possibly higher.
High brightness and colour performance are important because a digital cinema projector has to project images according to the DCI standard, including for instance a typical wider colour gamut.
In markets outside digital cinema a different colour gamut of the projector can be set such as to the REC709 colour gamut. But it is very important to mention that REC709 is only a recommendation, not a standard. Therefore the colour performance of the projectors can vary widely for instance for the colour point of the primaries, their colour to white ratios and the white colour point. The DCI standard is much stricter and defines the colour gamut and the white point of a Digital Cinema projection system. Some tolerances are allowed via narrow tolerance boxes expressed in a colour diagram for the white colour point and the colour points of the primary colours.
A comparison between the REC709 colour gamut and the DCI colour gamut is presented in FIG. 1.
Current laser-phosphor 3-chip projectors generate red green and blue primaries using blue lasers to excite a phosphor wavelength convertor and to generate yellow light. Direct blue laser light is added to the phosphor yellow light to create a white source. Blue lasers are preferred instead of blue LED's for the phosphor excitation due to the smaller etendue of laser light. Sometimes, additional red lasers or red LEDs are added to improve the red content. The typical optical spectrum of such a white illumination source consisting of direct blue lasers and yellow phosphor is presented in FIG. 2.
The colour point for the white laser+phosphor light source will vary due to a number of design choices. Additionally, with regard to the blue primary color point, for direct blue lasers the wavelength can vary in the interval 440 nm and 470 nm and one wavelength or a combination of different wavelengths can be used in this interval. The wavelength of the blue lasers can have some impact on the white point although their intensity or power level has much more impact. The selection of the blue laser has an impact on the location of the blue primary color point, in other words the left bottom corner of the color gamut.
The blue laser+yellow phosphor architecture has become very popular for projectors in the markets outside digital cinema due to its reduced complexity and right balance between performance and cost. The wavelength convertor for example is only one type of phosphor used to create both the red and the green component. Moreover, yellow phosphors with very good performance (e.g. high conversion efficiency, chemically stable, good quenching performance etc.) are readily available and the most popular example is the YAG:Ce phosphor used in white LEDs for lighting and backlighting applications. It is a well-known fact however that the application of red phosphors is not simple mainly due to the fact that red phosphors have poor thermal behaviour and they quench at temperatures much lower than those observed for good yellow phosphors. Also, the conversion efficiency of the red phosphors is much lower than that of a yellow phosphor (e.g. 30-35% compared with 60-65%). Hence having a good performing yellow phosphor with a significant red content has become in many cases the solution of choice.
However, for DCI compliant projectors, this very popular solution of only using blue lasers and a yellow phosphor proved to be rather limiting and additional improvements are required.
In order to have a DCI compliant projector when using such a white direct blue laser+yellow phosphor source a number of steps need to be carried out.
The first step is to achieve the native red, green and blue primaries according to the DCI spec. For most of the 3-chip projectors the splitting of the light generated by the light source into the three primaries, happening in the imaging module of the projector, is done by the Philips prism as seen in the FIG. 3. The Philips prism is also responsible for the initial filtering of the light. This filtering is a result of the typical difference of Angle Of Incidence (AOI) on the Philips prism coatings for incoming and outgoing light. The exactly impacted wavelength ranges are depending on the coating design but a typical case is that a dip around 490-500 nm (less visible in FIG. 3) and around 575-600 nm are created, for example.
However, the red and green primaries obtained in this way are still too broad to be DCI compliant. The colour points are not in the corresponding DCI tolerance boxes. An additional filtering in the green-red transition interval done with a notch filter is needed with the effect schematically represented in FIG. 4.
The wavelength interval between the green and red wavelengths (hence yellow wavelengths) where the imaging engine does the split-off between red and green results in a substantial amount of light loss.
The notch filter effect shown in FIG. 4 is just an example. In reality, the characteristics of the filter will have to be tailored to the exact phosphor spectrum and the exact specifications of the dichroic filters in the prism in order to correct the colour points of the primaries to be DCI compliant.
Due to the big difference between the optical spectrum of a Xenon lamp and a yellow phosphor, the light losses due to the use of a notch filter with a Xenon lamp are very different to that for a blue laser+yellow phosphor white source. In the case of the Xenon lamp this is typically 8% (in lumens). While in the case of blue laser+yellow phosphor white source this is approximately 18% (in lumens).
In addition to this significant decrease in brightness due to the notch filter for the specific case of a laser phosphor light source, another source of brightness reduction is the lack of red light and the excess of green light in the spectrum.
As a consequence, the major problem when using the blue laser+yellow phosphor architecture for a DCI compliant projector (in addition to the significant decrease in brightness due to the notch filter) is the lack of red light and the excess of green light in the typical spectrum of a yellow phosphor. Whereas this might not be a problem for projectors where the colour to white ratio is not a critical parameter, it's a major problem for DCI compliant projectors where the white color point (and therefore the red to white ratio) is very well defined in the DCI standard.
To solve this problem and bring the white color point at the DCI target value the excess of green light (and possibly blue also) has to be removed electronically. The same procedure is also used in current Xenon and Mercury lamp based projectors. But the losses due to these electronic corrections in the laser-phosphor based projectors are much higher than what is typically the case for a Xenon or Mercury lamp based projector. With typical values of 30% decrease in brightness due to the electronic correction, having a phosphor with such a limited red content proves to be a very serious issue.
To tackle the lack of red in the yellow phosphor spectrum, a solution typically called “red assisted laser phosphor source” has been proposed. In this case, an additional light source (direct red laser or red LEDs) is used to boost the red colour being produced. This additional light source is added to the existing blue laser+yellow phosphor solution without typically changing the type of phosphor that is used.
This is a very good solution in order to boost the red content and reduce the losses due to colour correction but it is still not minimizing the possible loses in brightness needed to achieve the DCI specification or other wide color gamut specifications or another standardized white point, or a combination of these.